Engine Time and Airframe Time do not report the same numbers

This is because the two times are calculated in different ways.

In the Maintenance Report you find the airframe total time (Hobbs Time) and the engine’s total time (Tach Time). The two value may slightly differ after loading a brand new airplane and then flying it for a while. This is because there’s an important difference in how the two times are calculated.

Hobbs Time

The Hobbs clock starts when the engine starts, and stops when the engine is shut-down. While running, it counts the time like a normal clock.

Tach Time

The Tach clock starts when the engine start, and stops when the engine is shut-down. While running, it counts the engine revolutions. While at about 70% of power, it counts the time like a normal clock. If the engine revolutions go down, the clock counts slower. If the engine revolutions go up, it counts faster.